Retinol-binding protein plasma concentrations

Cho, Y. M., Youn, B. S., Lee, H. et al. 2006. Plasma retinol-binding protein-4 concentrations are elevated in human subjects with impaired glucose tolerance and type 2 diabetes. 

Plasma Retinol Binding Protein Measurement of plasma concentrations of RBP may give some additional information. Indeed, it has been suggested that because retinol is susceptible to oxidation on storage of blood samples, measurement of RBP may be a better indication of the state of vitamin A status. In adequately nourished subjects, about 13% of immunologi-caUy reactive RBP in plasma is present as the apo-protein, whereas in vitamin A-deficient children, the proportion of apo-protein may rise to 50% to 90% of... 

For the diagnosis of hypervitaminosis A, plasma concentrations and retinol-binding protein may be misleading. Cases have been reported of hepatic fibrosis, secondary to chronic ingestion of massive doses of vitamin A, where plasma concentrations of vitamin A and retinol-binding protein at the time of diagnosis were within the reference range (43). 

There are also changes in the binding proteins in plasma as a result of the disease process. Since serum albumin falls in association with any acute iUness, this inevitably leads to a fall in plasma zinc concentration. Similarly a reduction in retinol-binding-protein concentration as part of the APR or protein malnutrition also leads to a fall in serum retinol levels, whatever the amount of retinol stores within the liver. 

Transthyretin amyloidosis (also called familial amyloid polyneuropathy) is an autosomal dominant syndrome characterized by peripheral neuropathy. This disease results from one of five mutations identified thus far in the gene for transthyretin. Transthyretin is also called prealbumin (although it has no structural relationship to albumin) because it migrates ahead of albumin in standard electrophoresis at pH 8.6. Transthyretin is synthesized in the liver and is a normal plasma protein with a concentration of 20-40 mg/dL. It transports thyroxine and retinol binding protein (Chapter 38). The concentration of transthyretin is significantly decreased in malnutrition and plasma levels are diagnostic of disorders of malnutrition (Chapter 17). 

Hydrophobic compounds are transported in plasma bound to transport proteins (e.g. the plasma retinol binding protein section 11.2.2.2) or dissolved in the lipid core of plasma lipoproteins (section 5.6.2), and net intracellular accumulation to a higher concentration than in plasma depends on an intracellular binding protein that has a greater affinity for the ligand than does the plasma transport protein. 

Waits RP, Yamada T, Uemichi T, Benson MD (1995) Low plasma concentrations of retinol-binding protein in individuals with mutations affecting position 84 of the transthyretin molecule. Clin Chem 41 1288-1291... 

Large S, Neal G, Glover J, Thanangkul O, Olson RE (1980) The early changes in retinol-binding protein and prealbumin concentrations in plasma of protein-energy malnourished children after treatment widi retinol and an improved diet. Brit JNutr 43 393-402... 

In 3 patients with chronic hypervitaminosis A increased plasma concentrations of total vitamin A and particularly of retinyl esters were seen. The concentration of plasma retinol-binding protein and pre-albumin... 

In malnutrition, total serum protein, albumin, and p-glob-uhn concentrations are reduced. The increased concentration of Y-globuIin does not fully compensate for the decrease in other proteins. The concentrations of complement C3, retinol-binding globulin, transferrin, and prealbumin decrease rapidly with the onset of malnutrition and are measured to define the severity of the condition. The plasma concentrations of lipoproteins are reduced, and serum cholesterol and triglycerides may be only 50% of the concentrations in healthy individuals. In spite of severe malnutrition, glucose concentration is maintained close to that in healthy... 

Oxidation of retinol produces retinoic acid tretinoin). The reaction is irreversible. Retinoic acid enters the portal blood, is transported bound to albumin, and is not stored to any great extent. The concentration of retinoic acid in plasma is normally 3-4 ng/mL. A biologically active metabolite, 5,6-epoxyretinoic acid, has been isolated from the intestinal mucosa of vitamin A-deficient rats following administration of H-retinoic acid. Several tissues have specific cellular retinoic acid-binding proteins (CRABPs). 

The daily requirement of vitamin A (Table 6.3) is provided to an extent of 75% by retinol intake (as fatty acid esters, primarily retinyl palmitate), while the remaining 25% is through P-carotene and other provitaminactive carotenoids. Due to the limited extent of carotenoid cleavage, at least 6 g of P-carotene are required to yield 1 g retinol. Vitamin A absorption and its storage in the fiver occur essentially in the form of fatty acid esters. Its content in fiver is 250pg/g fresh tissue, i.e. a total of about 240-540 mg is stored. The fiver supplies the blood with free retinol, which then binds to proteins in blood. The plasma concentration of retinol averages 1.78 pmol/1 in women and 2.04 lamol/l in men. 

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